Integral slide valve relief valve

ABSTRACT

Various embodiments of the invention provide a screw compressor for compressing a refrigerant in a refrigeration system. The screw compressor includes a rotor case, which accommodates a primary helical rotor that is intermeshed with at least one secondary helical rotor, to compress the refrigerant in the rotor case. The rotor case also includes a slide valve supporting member that is fixed on an inner wall of the rotor case. Moreover, a slide valve, located in the rotor case, slides axially on the slide valve supporting member. The slide valve controls the volume of refrigerant during compression. The slide valve is fitted with at least one internal pressure relief valve, to relieve internal pressure in the screw compressor.

BACKGROUND

The invention generally relates to screw compressors. More specifically,the invention relates to positioning an internal pressure relief valvein a screw compressor.

A screw compressor, in addition to a condenser, an expansion device, andan evaporator, is a key component of a refrigeration system. The screwcompressor includes a common housing, hereinafter referred to as a‘rotor case’, in which a primary helical rotor is intermeshed with atleast one secondary helical rotor. The primary helical rotor is drivenby an electric motor. Due to the intermeshing between the primary andsecondary helical rotors, the secondary helical rotor is driven incounter-rotating motion by the rotation of the primary helical rotor.The function of the primary and secondary helical rotors is to compressa refrigerant entering the screw compressor in a gaseous state,hereinafter referred to as a ‘refrigerant gas’. A refrigerant is amedium of heat transfer that produces a refrigeration effect byabsorbing and dissipating heat in the refrigeration system. Therefrigeration effect refers to the amount of cooling produced in therefrigeration system. The refrigerant gas enters the rotor case throughan inlet port and gets trapped between the inner walls of the rotor caseand grooves of the primary and secondary helical rotors. Due to theconstant rotary motion of the primary and secondary helical rotors, therefrigerant gas gets compressed and is discharged through a dischargeport. The compressed refrigerant gas then enters the condenser underhigh pressure. In the condenser, the refrigerant gas is cooled andthereafter liquefied by heat exchange with the air or water present inthe condenser. Thereafter, the resulting refrigerant liquid is expandedin the expansion device and is brought down to a low pressure andtemperature. The low pressure, low temperature refrigerant liquid isthen supplied to the evaporator. In the evaporator, the refrigerantliquid absorbs the heat present in the evaporator and changes intogaseous state, thereby cooling the refrigeration system. Subsequently,the refrigerant gas leaves the evaporator and undergoes compression inthe screw compressor, thus completing a refrigeration cycle.

To vary the refrigeration effect, the compression capacity of therefrigeration system needs to be controlled. The compression capacityrefers to the volume of compressed refrigerant gas discharged from thescrew compressor. The compression capacity is proportional to therefrigeration effect in the refrigeration system. In order to controlthe compression capacity, the rotor case of the screw compressor isprovided with a slide valve. The slide valve controls the compressioncapacity by varying the volume of the refrigerant gas in a workingchamber. The working chamber is defined by the inner walls of the rotorcase of the screw compressor. This allows only the required volume ofrefrigerant gas to be compressed and discharged from the screwcompressor.

During the process of compression, an over-pressure situation may arisein the screw compressor. During the over-pressure situation, theinternal pressure of the screw compressor exceeds the maximum allowableinternal pressure. This can cause damage to the screw compressor. Inorder to relieve excess internal pressure, an internal pressure reliefvalve is provided in the screw compressor. The internal pressure reliefvalve vents the excess internal pressure from the discharge side(high-pressure side) to the suction side (low-pressure side) of thescrew compressor, thus preventing damage to the screw compressor.

FIG. 1 illustrates a view of a conventional screw compressor 100 withits components. Screw compressor 100 includes a rotor case 102, a slidevalve supporting member 108, a slide valve 110, and two internalpressure relief valves 112 a and 112 b. Rotor case 102 accommodates aprimary helical rotor 104 and a secondary helical rotor 106. Primaryhelical rotor 104 and secondary helical rotor 106 together compress therefrigerant gas in rotor case 102. Conventionally mounted internalpressure relief valves 112 a and 112 b take up a lot of space in screwcompressor 100. Further, when the size of screw compressor 100increases, either the size of internal pressure relief valves 112 a and112 b have to be increased accordingly, or more internal pressure valveshave to be accommodated in rotor case 102 of screw compressor 100. As aresult, the amount of casting material required to manufacture rotorcase 102 increases, thereby increasing the overall size, weight, andmanufacturing cost of screw compressor 100.

In light of the foregoing discussion, there exists a need for anapparatus that relieves excess internal pressure in a screw compressor,without taking up any additional space, even when the size of the screwcompressor is increased or multiple internal pressure valves have to beaccommodated. Further, the apparatus should be arranged such that anyextra amount of casting material or additional cost of manufacturing therotor case of the screw compressor is avoided.

SUMMARY

An object of the invention is to provide a screw compressor forcompressing a refrigerant in a refrigeration system.

Another object of the invention is to provide an apparatus for relievingexcess internal pressure in the screw compressor.

Yet another object of the invention is to provide an apparatus whichdoes not take up any additional space, even when the size of the screwcompressor is increased or multiple internal pressure relief valves haveto be accommodated

Still another object of the invention is to provide a screw compressorthat reduces the casting material and cost of manufacturing a rotor caseof the screw compressor.

Still another object of the invention is to provide an apparatus thatreduces the overall size and weight of the screw compressor, withoutchanging the functionality of the screw compressor.

In order to achieve the above-mentioned objects, the invention providesa screw compressor that comprises a rotor case and an electric motor.The rotor case includes a primary helical rotor that is intermeshed withat least one secondary helical rotor, to compress a refrigerant in therotor case. The rotor case also comprises an inlet opening for therefrigerant to enter the screw compressor. The refrigerant from theinlet opening enters the suction ports. Further, the refrigerant fromthe suction ports enters a working chamber, which is defined by spacebetween the inner walls of the rotor case. A slide valve supportingmember is fixed on the inner wall of the rotor case. The rotor case alsoaccommodates a slide valve, which slides axially on the slide valvesupporting member. The slide valve is fitted with at least one internalpressure relief valve that relieves excess internal pressure in thescrew compressor.

In the screw compressor, the refrigerant enters the working chamberthrough the suction ports. The electric motor drives the primary helicalrotor, which, in turn, drives the secondary helical rotor in a counterrotating motion. Due to the constant rotary motion, the refrigerant getstrapped between the helical grooves of the primary and the secondaryhelical rotors and is compressed to high pressure. The slide valvelocated in the rotor case provides compression capacity control byvarying the volume of the refrigerant in the working chamber. In anembodiment of the invention, the compression capacity of the screwcompressor can be decreased by exposing a portion of the working chamberto one of the suction ports. This vents a required volume of therefrigerant gas into one of the suction ports. Hence, only the remainingvolume of the refrigerant in the working chamber is compressed. Theinternal pressure relief valves fitted in the slide valve, relieve theexcess internal pressure in the screw compressor during over-pressuresituations. The over-pressure situation occurs when the internalpressure of the screw compressor exceeds the maximum allowable internalpressure of the screw compressor. During over-pressure situations, theinternal pressure relief valves relieve the excess internal pressurefrom the discharge side to the suction side, thereby preventing damageto the screw compressor. Moreover, by fitting the internal pressurerelief valves in the slide valve, the amount of casting material and thecost of manufacturing the rotor case reduces, resulting in a reductionof the overall size and weight of the screw compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will hereinafter be described inconjunction with the appended drawings, provided to illustrate and notto limit the invention, wherein like designations denote like elements,and in which:

FIG. 1 illustrates a view of a conventional screw compressor with itscomponents;

FIG. 2 illustrates a block diagram depicting the components of arefrigeration system, in accordance with an embodiment of the invention;

FIG. 3 illustrates a view of a screw compressor, showing two internalpressure relief valves mounted in a slide valve, in accordance with anembodiment of the invention;

FIG. 4 illustrates a view of a screw compressor, showing three internalpressure relief valves housed in a slide valve, in accordance with anembodiment of the invention;

FIG. 5 illustrates a block diagram depicting various components of ascrew compressor, in accordance with an embodiment of the invention; and

FIG. 6 illustrates a cross sectional view of a valve with its componentsfor relieving internal pressure in a screw compressor, in accordancewith an embodiment of an invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Various embodiments of the invention relate to a screw compressor forcompressing a refrigerant in a refrigeration system. More specifically,the embodiments relate to the positioning of internal pressure reliefvalves in the screw compressor. The positioning of these internalpressure relief valves depends on various factors such as the size andweight of the screw compressor, the amount of casting material required,and the cost of manufacturing the rotor case of the screw compressor.

FIG. 2 illustrates a block diagram depicting the components of arefrigeration system 200, in accordance with an embodiment of theinvention. Refrigeration system 200 includes a screw compressor 202, acondenser 204, an expansion device 206, and an evaporator 208. Screwcompressor 202 can be an oil type compressor, an oil-free typecompressor, and the like. An oil type screw compressor utilizes oil forlubricating the bearings and for cooling the internal components of thescrew compressor. Connectors 210 a, 210 b, 210 c, and 210 d, hereinafterreferred to as ‘connectors 210’, indicate the direction in which arefrigerant flows from one component to another in refrigeration system200. A refrigerant is a medium of heat transfer that absorbs heat fromevaporator 208 and dissipates heat at condenser 204, thereby coolingrefrigeration system 200. The refrigerant can be ammonia, Freon,halocarbons, hydrocarbons, and the like. The refrigerant enters screwcompressor 202 in a gaseous state, hereinafter referred to as‘refrigerant gas’. The refrigerant gas gets compressed to a highpressure in screw compressor 202. The resulting high-pressurerefrigerant gas enters condenser 204, where it is liquefied by heatexchange with the air or water present in condenser 204. Thereafter, theresulting refrigerant liquid is expanded in expansion device 206 and isbrought down to a low pressure and temperature. The low-pressure,low-temperature refrigerant liquid is then supplied to evaporator 208.In evaporator 208, the refrigerant liquid absorbs the heat present inevaporator 208 and changes into a gaseous state, thereby coolingrefrigeration system 200. Subsequently, the refrigerant gas thus formedleaves evaporator 208 and is compressed in screw compressor 202, therebycompleting a refrigeration cycle.

FIG. 3 illustrates a view of screw compressor 300, showing two internalpressure relief valves 112 a and 112 b mounted in slide valve 110, inaccordance with an embodiment of the invention. Screw compressor 300includes a rotor case 102, a slide valve supporting member 108, a slidevalve 110, and two internal pressure relief valves 112 a and 112 b.Rotor case 102 accommodates a primary helical rotor 104 and a secondaryhelical rotor 106. Primary helical rotor 104 and secondary helical rotor106 together compress the refrigerant gas in rotor case 102. Internalpressure relief valves 112 a and 112 b are mounted in slide valve 110 torelieve excess internal pressure from the discharge side to the suctionside of screw compressor 300, thereby preventing damage to screwcompressor 300. The suction side refers to the inlet side through whichthe refrigerant gas enters screw compressor 300 at suction pressure. Thedischarge side refers to the outlet side through which the refrigerantgas is discharged out of screw compressor 300 at discharge pressure.

Conventionally mounted internal pressure relief valves 112 a and 112 btake a lot of space in screw compressor 300. This increases the amountof casting material required and the cost of manufacturing rotor case102 of screw compressor 300. The casting material can be cast steel,gray cast iron casing, nodular iron casing, and the like. Embodiments ofthe invention describe mounting internal pressure relief valves 112 aand 112 b in slide valve 110 of screw compressor 300. This reduces theamount of casting material required and the cost of manufacturing rotorcase 102. The reduction in the amount of casting material required andthe cost of manufacturing rotor case 102 does not affect the normalfunctioning of slide valve 110, internal pressure relief valves 112 aand 112 b, and screw compressor 300.

FIG. 4 illustrates a view of screw compressor 400, showing threeinternal pressure relief valves 416 a, 416 b, and 416 c housed in slidevalve 410, in accordance with an embodiment of the invention. Screwcompressor 400 includes a rotor case 402, a primary helical rotor 404, asecondary helical rotor 406, a slide valve supporting member 408, aslide valve 410, a piston 412, and a piston rod 414. Primary helicalrotor 404 is intermeshed with secondary helical rotor 406, to trap andcompress the refrigerant gas between the inner walls of rotor case 402and the helical grooves of primary helical rotor 404 and secondaryhelical rotor 406. Slide valve supporting member 408 is fixed on aninner wall of rotor case 402. Slide valve 410 moves axially on slidevalve supporting member 408. Piston rod 414 is connected to slide valve410 on one side and to piston 412 on the other.

Further, it should be noted that screw compressor 400 can include morethan one secondary helical rotor. All the secondary helical rotors canbe intermeshed with the primary helical rotor 404. Although theinvention has been discussed with respect to specific embodimentsthereof, these embodiments are merely illustrative, and not restrictive,of the invention.

FIG. 5 illustrates a block diagram showing various components of a screwcompressor 500, in accordance with an embodiment of the invention. Screwcompressor 500 can be a hermetic screw compressor, a semi-hermetic screwcompressor, and the like. A hermetic screw compressor is a completelysealed and airtight screw compressor. Screw compressor manufacturersmake hermetic or semi-hermetic screw compressors to achieve higherefficiency, minimum leakage, ease of service, and volume production (ofdischarged refrigerant gas). The selection of a hermetic orsemi-hermetic screw compressor depends on factors such as the area ofapplication, the refrigerant to be used, and the manufacturer of thescrew compressor.

Screw compressor 500 includes a rotor case 502, and a bearing case 504.Rotor case 502 includes a primary helical rotor 506, a secondary helicalrotor 508, a bearing 510 a, an inlet opening 514, suction ports 516 aand 516 b, a slide valve 518, a slide valve supporting member 520, andtwo internal pressure relief valves 528 a and 528 b. Bearing case 504includes a bearing 510 b, a hydraulic cylinder 522, a piston 524, apiston rod 526, hydraulic fluid apertures 530 and 532, a dischargechamber 534, and a discharge outlet 536. An electric motor 512 and anoil-separating module 538 are connected to screw compressor 500.

In rotor case 502, primary helical rotor 506 is intermeshed withsecondary helical rotor 508. In rotor case 502, a shaft of primaryhelical rotor 506 is supported by bearing 510 a. Similarly, in bearingcase 504, a shaft of secondary helical rotor 508 is supported by bearing510 b. The shaft of primary helical rotor 506 is coupled to a primemover such as electric motor 512, which drives primary helical rotor506. In an embodiment of the invention, electric motor 512 can be placedinside rotor case 502. In another embodiment of the invention electricmotor 512 can be placed outside rotor case 502. Due to the intermeshingbetween primary helical rotor 506 and secondary helical rotor 508,secondary helical rotor 508 is driven in a counter-rotating motion byprimary helical rotor 506.

Further, slide valve 518 in the inner wall of rotor case 502 controlsthe compression capacity of screw compressor 500. The compressioncapacity of screw compressor 500 refers to the volume of refrigerant gasbeing compressed and discharged from screw compressor 500. Slide valve518 slides axially on slide valve supporting member 520, which is fixedon the inner wall of rotor case 502. Slide valve 518 is driven by piston524, which is accommodated in hydraulic cylinder 522. Piston rod 526connects slide valve 518 and piston 524. Hydraulic cylinder 522, piston524 and piston rod 526, together, constitute a driving module. Thedriving module facilitates driving slide valve 518 axially.

The refrigerant gas leaving the evaporator enters screw compressor 500through inlet opening 514. The refrigerant gas flows into suction port516 a and enters a working chamber, which is defined by the spacebetween inner walls of rotor case 502. Primary helical rotor 506 andsecondary helical rotor 508 rotate in the working chamber to compressthe refrigerant gas to high pressure. Further, the compressedrefrigerant gas flows from the working chamber and gets collected indischarge chamber 534 of bearing case 504. Thereafter, the refrigerantgas is forwarded to oil-separator module 538 located outside screwcompressor 500 through discharge outlet 536. In an embodiment of theinvention, oil-separator module 538 can be located inside bearing case504 before discharge chamber 534. In another embodiment of theinvention, oil-separator module 538 can be placed outside screwcompressor 500 after discharge outlet 536. In oil-separator module 538,the oil present in the compressed refrigerant gas is separated. Theresulting compressed refrigerant gas which is free from oil enterscondenser 204.

Screw compressor 500 can be an oil type screw compressor, oil-free typescrew compressor, and the like. Oil type screw compressors utilize oilto lubricate the bearings, to cool the internal components, and to sealthe spaces between the inner walls of the rotor case and the helicalrotors. For example, if screw compressor 500 is oil type, then oil isutilized to lubricate bearings 510 a and 510 b, to cool the internalcomponents and to seal the spaces between the inner walls of rotor case502 and helical rotors 506 and 508. Screw compressors use hydraulicfluids to actuate the piston, to drive the slide valve axially. Thehydraulic fluid can be oil, refrigerant gas, and the like. For example,if screw compressor 500 is oil type, oil is used to actuate piston 524.Similarly, an oil-free type compressor uses the refrigerant gas toactuate the piston. Therefore, to actuate piston 524, hydraulic cylinder522 is provided with two hydraulic fluid apertures 530 and 532, for thehydraulic fluid to enter hydraulic cylinder 522.

The position of slide valve 518, covering suction port 516 b, isreferred to as a ‘full-load position’. In the full-load position, thevolume of refrigerant gas entering screw compressor 500 through inletopening 514 is fully utilized during compression. In order to vary theload position, slide valve 518 needs to be actuated so that it uncoverssuction port 516 b and exposes the working chamber of rotor case 502 tosuction port 516 b. To achieve this, hydraulic cylinder 522 is suppliedwith oil through aperture 530. To vary the load position, piston 524moves slide valve 518 in the direction of arrow 540. This exposes theworking chamber to suction port 516 b, thereby venting the refrigerantgas present in the working chamber to suction port 516 b. Further thevolume of refrigerant gas present in the working chamber varies.Consequently, the compression capacity of screw compressor 500 varies.In an embodiment of the invention, if the volume of refrigerant gaspresent in the working chamber is decreased, the compression capacity ofscrew compressor 500 is reduced. In another embodiment of the invention,if the compression capacity of screw compressor 500 needs to beincreased, hydraulic cylinder 522 is supplied with hydraulic fluidthrough aperture 532. As a result, piston 524 pushes slide valve 518 inthe direction of arrow 542, covering suction port 516 b. This isfollowed by the compression process, thereby increasing the compressioncapacity of screw compressor 500.

During the process of compression in screw compressor 500, the internalpressure of screw compressor 500, on the discharge side, should not beallowed to exceed the maximum allowable internal pressure of screwcompressor 500. This situation in which the internal pressure does notexceed the maximum allowable internal pressure of screw compressor 500is referred to as ‘normal-pressure situation’. Further, a situation inwhich the internal pressure exceeds the maximum allowable internalpressure of screw compressor 500 is referred to as ‘over-pressuresituation’. The over-pressure situation can damage screw compressor 500.To avoid this damage, slide valve 518 is fitted with two internalpressure relief valves 528 a and 528 b, hereinafter referred to as‘valve 528’ which relieve pressure at a pre-determined setting (pressuredifference).

FIG. 6 illustrates a cross sectional view of a valve 528 with itscomponents for relieving internal pressure in a screw compressor, inaccordance with an embodiment of the invention. Valve 528 comprises avalve body 602. Valve body 602 acts as housing for the internalcomponents of valve 528. Valve body 602 includes a valve spring 604, avalve ball 606, a valve inlet 608 (on the discharge side), and a valveoutlet 610 (on the suction side). During normal-pressure situations,valve ball 606 closes valve inlet 608 from inside, which does not allowthe refrigerant gas to enter valve 528. This is due to the actuatingforce exerted on valve ball 606 by valve spring 604. The actuating forceis exerted by valve spring 604 in the direction indicated by an arrow612. This position is referred to as a ‘closed position’ of valve 528.During over-pressure situations, the refrigerant gas at the dischargeside forces valve ball 606 against the actuating force of valve spring604. The direction of force exerted on valve ball 606 against theactuating force of valve spring 604 is indicated by an arrow 614.Consequently, the refrigerant gas enters through valve inlet 608 andleaves through valve outlet 610, thereby relieving the excess internalpressure from the discharge side to the suction side of screw compressor500. Further, when the internal pressure at the discharge side dropsbelow the maximum allowable internal pressure of screw compressor 500,normal-pressure situation arises. During normal-pressure situation,valve spring 604 exerts the actuating force on valve ball 606.Consequently, valve ball 606 is pushed back to close valve inlet 608,thus bringing back valve 528 to the closing position.

In accordance with another embodiment of the invention, valve ball isreplaced by a valve piston. The valve piston includes a valve seat thatis attached to first end of the valve piston. The second end of thevalve piston is attached to the valve spring. The valve seat can be madeof a soft material such as Teflon, and the like. The valve seat rests onto the inner surface of the valve inlet. The valve piston and the valveseat act as a closing assembly, thereby preventing the refrigerant gasfrom entering through the valve inlet during normal-pressure situations.During over-pressure situations, the refrigerant gas pushes the valvepiston against the actuating force of the valve spring. As a result, therefrigerant gas enters valve 528 through the valve inlet, therebyrelieving the excess pressure from the discharge side to the suctionside. Further, during normal-pressure situations, the actuating force ofvalve spring acts on the valve piston. The valve piston is pushed backto close the valve inlet, thus disallowing the refrigerant gas fromentering valve 528 through the discharge side.

Various embodiments of the invention provide a screw compressor forcompressing a refrigerant in a refrigeration system. Such a screwcompressor provides compression capacity control and relieves excessinternal pressure during over-pressure situations, thereby preventingthe screw compressor from being damaged. Further, various embodiments ofthe invention provide a cost-effective screw compressor in which theinternal pressure relief valves are mounted in the slide valve, therebyreducing the amount of casting material required and the cost ofmanufacturing the rotor case, as well as the overall weight and size ofthe screw compressor.

While the various embodiments of the invention have been illustrated anddescribed, it will be clear that the invention is not limited to theseembodiments only. Numerous modifications, changes, variations,substitutions and equivalents will be apparent to those skilled in theart, without departing from the spirit and scope of the invention, asdescribed in the claims.

1. An apparatus for relieving pressure in a screw compressor, the screwcompressor comprising a rotor case, the rotor case comprising aplurality of helical rotors, the plurality of helical rotors comprisinga primary helical rotor and at least one secondary helical rotor, theprimary helical rotor intermeshed with each of the at least onesecondary helical rotor for compressing a refrigerant, the apparatuscomprising: a slide valve supporting member fixed on an inner wall ofthe rotor case; a slide valve positioned between the plurality ofhelical rotors and the slide valve supporting member, the slide valvesliding axially on the slide valve supporting member, the slide valvecontrolling the volume of refrigerant to be compressed; and at least oneinternal pressure relief valve positioned to relieve said pressure froma discharge side of the compressor to a suction side of the compressor,each of the at least one internal pressure relief valve being fitted inthe slide valve, each of the at least one internal pressure relief valverelieving pressure in the screw compressor, the pressure being relievedbased on a pre-determined setting.
 2. The apparatus according to claim1, wherein each of the at least one internal pressure relief valvecomprises: a valve inlet, the valve inlet providing an opening to therefrigerant to enter the each of the at least one internal pressurerelief valve; a valve ball, the valve ball closing and opening the valveinlet; and a valve spring, the valve spring moving during the openingand closing of the valve inlet, wherein one end of the valve spring isconnected to the valve ball.
 3. The apparatus according to claim 1,wherein each of the at least one internal pressure relief valvecomprises: a valve inlet, the valve inlet providing an opening to therefrigerant to enter the each of the at least one internal pressurerelief valve; a valve piston, the valve piston closing and opening thevalve inlet; a valve seat, one end of the valve seat connected to afirst end of the valve piston, wherein the valve seat rests on thesurface of the valve inlet when the valve piston closes the valve inlet;and a valve spring, one end of the valve spring connected to a secondend of valve piston, the valve spring moving during the opening andclosing of the valve inlet.
 4. The apparatus according to claim 1,wherein each of the at least one internal pressure relief valve ishoused in the slide valve.
 5. The apparatus according to claim 1,wherein each of the at least one internal pressure relief valve ismounted in the slide valve.
 6. The apparatus according to claim 1,wherein the pre-determined pressure difference being the internalpressure of the screw compressor exceeding the maximum allowableinternal pressure of the screw compressor.
 7. A screw compressor forcompressing a refrigerant in a refrigeration system, the screwcompressor comprising: a rotor case; a plurality of helical rotors, theplurality of helical rotors being placed in the rotor case, theplurality of helical rotors comprising a primary helical rotor and atleast one secondary helical rotor, the primary helical rotor beingintermeshed with each of the at least one secondary helical rotor; aslide valve controlling the volume of refrigerant to be compressed; andat least one internal pressure relief valve positioned to relievepressure from a discharge side of the compressor to a suction side ofthe compressor, each of the at least one internal pressure relief valvebeing fitted in the slide valve for relieving pressure from the screwcompressor, the pressure being relieved based on a pre-determinedsetting.
 8. The screw compressor according to claim 7, wherein one ofthe plurality of helical rotors is connected to a prime mover, the primemover driving the one of the plurality of helical rotors.
 9. The screwcompressor according to claim 7, wherein the slide valve is connected toa driving module, the driving module moving the slide valve axially. 10.The screw compressor according to claim 9 comprising: a hydrauliccylinder; a piston, the piston accommodated in the hydraulic cylinder;and a piston rod, the piston rod comprising: a first end, the first endconnected to the slide valve; and a second end, the second end connectedto the piston.
 11. The oil type screw compressor according to claim 7using oil for cooling and sealing a space between the plurality ofhelical rotors and the inner wall of the rotor case.
 12. The screwcompressor according to claim 7 being connected to an oil-separatingmodule, the oil-separating module separating oil from the compressedrefrigerant.
 13. The screw compressor according to claim 7 being anoil-free screw compressor.
 14. The screw compressor according to claim 7being a hermetic screw compressor.
 15. The screw compressor according toclaim 7 being a semi-hermetic screw compressor.
 16. A system forcompressing a refrigerant in a refrigeration system, the systemcomprising: a plurality of helical rotors, the plurality of helicalrotors comprising a primary helical rotor being intermeshed with atleast one secondary helical rotor; an accommodating means for enclosingthe plurality of helical rotors; a slide valve supporting means beingfixed on an inner wall of the accommodating means; a sliding meanspositioned between the plurality of helical rotors and the slide valvesupporting means, the sliding means sliding axially on the slide valvesupporting means, the sliding means controlling the volume ofrefrigerant to be compressed; and a pressure relieving means fitted inthe sliding means and positioned to relieve pressure from a dischargeside of the compressor to a suction side of the compressor, the pressurebeing relieved based on a pre-determined setting.
 17. The systemaccording to claim 16, wherein the sliding means is connected to adriving module, the driving module moving the sliding means axially. 18.The system according to claim 17 wherein the driving module comprises: ahydraulic cylinder; a piston, the piston accommodated in the hydrauliccylinder; and a piston rod, the piston rod comprising: a first end, thefirst end connected to the sliding means; and a second end, the secondend connected to the piston.
 19. The system according to claim 16,wherein the pre-determined setting is a pre-determined pressuredifference.
 20. The apparatus according to claim 1, wherein thepre-determined setting is a pre-determined pressure difference.
 21. Thecompressor of claim 7 wherein: the pre-determined setting is apre-determined pressure difference.